The Diabetes and Womens Health Study, based on a retrospective cohort design, aims to understand pathways and determinants underlying the progression from gestational diabetes (GDM) to type 2 diabetes (T2DM) and complications . GDM is a common pregnancy complication. Women who develop impaired glucose tolerance and/or GDM in pregnancy are at substantially increased risk for T2DM in the years following pregnancy. Determinants underlying the transition from GDM to T2DM are not well studied and available studies are limited. There is limited information about the genetic and environmental factors that impact this transition in particular. Further, there is limited research aimed at following women with GDM long term through later adulthood for the development of T2DM and co-morbidities. These critical data gaps serve as the impetus for this study with the overall goal of investigating genetic factors and their interactions with risk factors amenable to clinical or public health intervention in relation to the transition of GDM to T2DM. Data collection for this study will build upon two large existing cohorts: the Nurses Health Study II (NHS-II) and the Danish National Birth Cohort (DNBC). In the present study, we will enroll approximately 4,500 women with a history of GDM who were members of either the NHS II or DNBC. After enrollment, we will follow participants for an additional four years to collect updated information on major clinical and environmental factors including, but not limited to, diet, physical activity and anthropometric information that may predict T2DM risk;to collect timed biospecimens including blood, urine and toenails to measure genetic and biochemical markers implicated in glucose metabolism.